Electrolytic polishing process



June 21, 1949. 2,473,923

D. R. TURNER ELECTROLYTIC POLISHING PROCESS Filed Jan. 13, 1945 A. Fey/027 of camp/eta po/Ls'b/fly /'77 30 seconds.

5. M fey/027 of mcamp/ete poms/7mg. mzm k g 6 k i 'i S F ,3 n gm Q R n k K) 0 a/ a2 0.3 04 a5 06 63/6/86 Per Second WITNESSES: INVENTOR Patented June 21, 1949 ELECTROLYTIC POLISHING PROCESS Dennis R. Turner, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 13, 1945, Serial No. 572,589

2 Claims.-

for cleaning and polishing silver and high silver alloy surfaces.

Silver, when cleaned and polished, presents a bright, attractive appearance, and such polished surfaces are necessary and desirable in apparatus, household ware and other devices embodying silver or silver alloys.

Numerous chemical and electrochemical processes for polishing silver and its alloys are known, but are characterized by excessive time required, costly ingredients, and, in some cases, the attainment of a surface whose reflectivity is far less than that considered a good polish. In particular, it is desirable to polish freshly electroplated silver to a high degree of brilliancy. A member freshly removed from a silver electroplating bath has a surface that is of a dull or matte appearance. Frequently, mechanical bumng and polishing are resorted to as the only suitable means for producing a brilliant reflective polished surface.

The object of this invention is to electropolish rapidly and conveniently silver and its alloys in an electrolyte by applying alternating current.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of this invention, reference should be had to the following detailed description and drawing, in which the sole figure is a graph plottinecurrent density against the frequency in cycles per second to carry out the invention.

According to the present invention, silver and its alloys are electropolished in a brief period of time by employing an alternating electrical current in combination with an alkaline cyanide electrolyte. By silver it is meant silver metal and alloys composed of silver as the main constituent. both plated and solid.

The electrolyte to be applied to the silver surfaces consists of at least 2 grams per liter of cyanide and suflicient of another more readily ionizable non-acidic compound to give good electrical conductivity to the solution. A solution composed of 32 grams per liter of potassium cyanide and 8 grams per liter of potassium hydroxide gave excellent results in practice. From 2 to 50 grams per liter of cyanide is suitable to accomplish the purpose of the invention. Various neutral salts and alkaline compounds may be added to the electrolyte in order to attain the desired electrical conductivity. As an example. with 32 grams per liter of potassium cyanide, 10 grams per liter of any of the following compounds gave excellent results:

Ammonium chloride Sodium chloride Sodium carbonate 2 Sodium bicarbonate Trisodium phosphate Sodium hydroxide Potassium hydroxide Potassium chloride Ammonium hydroxide The amount of the conducting compounds may be more or less than this amount. As much as grams per liter is not excessive while 5 grams perliter is operative. Obviously, compounds detrimental to silver should not be added to the electrolyte. Mixtures of the easily ionizable compounds may be employed for this purpose. The electrolyte after the additions of the ionizable compound and the cyanide is preferably alkaline in reaction. 7

For use in electropolishing, the electrolyte may be at any temperature up to the boiling point. Room temperatures are preferred although it will be found that the polishing proceeds somewhat faster at higher temperatures.

When the electrolyte has been employed for a considerable period of time, it will be found that the cyanide will become depleted, and the polishing action will take much longer to accomplish. This condition can be remedied by adding more cyanide, such for example, as sodium cyanide or potassium cyanide.

In cleaning solid or plated silver members, the members are connected to one terminal from a source of alternating current and immersed in a bath of the hot electrolyte prepared as described above. The other terminal from the source of alternating current may be connected either to a similar silver member or to an inert anode, such, for example, as nickel or nickel-plated steel. The nickel electrodes should be shaped or disposed to give a reasonable degree of uniformity of current over the surface of the silver member being electropolished since the throwing power of the bath is moderate.

It has been found that the frequency of the electrical current in combination with the current density bear a critical relationship for good polishing in reasonable lengths of time. For 60- cycle current, a current density of 5,000 amperes per square foot gives a brilliant polish in about 5 to 15 seconds. Current densities as low as 800 amperes per square foot produce good results, but take a slightly longer time to eifect complete polishing. As the current density at this frequency is reduced, it becomes more diflicult to achieve a complete polishing job no matter how prolonged the process. While higher frequencies may be used, the current densities required are high and no particular advantage is secured.

Much more economical results have been obtained by using currents of lower frequency for the polishing of silver members. Referring to the figure of the drawing, there is plotted the irequency in cycles per second against current density in amperes per square foot required to produce complete polishing of members in 30 seconds. Thus, with current of 0.1 cycle per second, a current density of about 140 amperes per square foot will produce an excellent polishing job in 30 seconds. Obviously, the time will be reduced as the current density at this frequency is increased. As the frequency increases, the current density required to effect an equally good polishing job in the same time is greater. In the region identified as A above the curve In in the graph of the drawing, electropolishing gives the most satisfactory results. For satisfactory electropolishing a current of at least 100 amperes per square foot is required at a frequency of 0.1 cycle per second, the minimum current density for an alternating electrical current of a frequency of between 0.1 and 0.6 cycle per second increasing from at least 100 amperes per square foot at 0.1 cycle per second, and following curve In in the drawing, and the minimum current density rising to a value of 800 amperes per square foot with 60 cycles per second electrical current. As the process is carried out in the region B under the curve I0, polishing will be effected mostly at the edges and corners of the silver member and will be unsatisfactory. Operating in area B, satisfactory polishing is not secured even when the time of treatment exceeds 30 seconds.

Where the current frequency is less than about 0.2 cycle per second, it has been found to be critical in securing a good polished product that the silver member be removed from the electrolyte on the deplating portion of the cycle, that is,

when the member is the anode. With higher frequency current, the portion of the cycle at which the member is removed from the electrolyte becomes of less importance.

For low frequency currents, as shown in the figure of the drawing, direct current or rectified alternating current may be reversed periodically by a suitable motor-operated reverse switching means. The voltage need only be two to three volts. In some cases, the voltage may be higher.

if the electrolyte is not sufficiently conducting or if the silver members are placed in series.

In operating the electropolishing process, the electrolyte appears to function to give a satisfactory polished silver surface on members in from 5 to 30 seconds, depending upon the temperature of the electrolyte, concentration of the cyanide, the current density, and the frequency of the electrical current. In some cases, a small amount of agitation of the electrolyte or electrodes has been found to be beneficial.

After the silver member has been removed from the electrolyte, it is rinsed thoroughly with water,

to remove the cyanide therefrom. The silver member is briefly immersed in a acid bath using sulfuric acid and rinsed again with water. The silver member may be dried and is immediately ready for use.

It will be discovered that the silver surface has been given a brilliant sparkling polish by the electrical process of this invention. No significant amount of metal will have been lost since the process plates back, when the member is the cathode, a substantial proportion of the metal that is removed from the member when it was the anode.

The process herein indicated may be applied 4 to household silverware, jawelry of all kinds, silver-plated apparatus, silver contacts, silver or silver-plated electrical resonators, and any similar type of silver or silver-plated device.

Since certain changes in carrying out the above process which embodies the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the method of polishing a member whose surface is composed of silver, the steps comprising immersing the member in an aqueous alkaline cyanide solution consisting of from 2 to 50 grams per liter of alkali cyanide and from 5 to grams per liter of another readily ionizable alkaline compound to provide for good electrical conductivity, passing an alternating=electrical current of a frequency of between 0.1 cycle and 0.6

cycle per second at a current density of not less than 100 amperes per square foot at 0.1 cycle per second, the current density for other frequencies between these limits being maintained in the area A in the figure of the drawing, the flow of current being applied for about 30 seconds, and withdrawing the member from the solution when the current cycle renders it anodic for frequencies between 0.1 and 0.2 cycle per second.

2. The method of polishing members whose surfaces are composed of silver, comprising, immersing the members in an aqueous electrolyte composed of from 2 to 50 grams per liter of alkali cyanide and from about 5 to 100 grams per liter of another readily ionizable, non-acidic compound, passing an alternating electrical current through the members having a frequency of from about 0.1 cycle to 60 cycles per second and at a current density of from at least 100 amperes'per square foot at 0.1 cycle per second to at'least 800 amperes per square foot at sixty cycles per second, up to a maximum of 5000 amperes per square foot applied for a period of from 5 to 30 seconds to electropolish the silver surfaces, and removing the member from the electrolyte when the current renders it anodic for frequencies of between 0.1 and 0.2 cycle per second.

DENNIS R. TURNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of The Electrochemical Socie yol. 81 (1942) pages 199 through 211.

The Metal Industry, April 1910, page June 1921, page 255.

Chemical Abstracts, vol. 37 (1943), pag

Protective Coatings for Metals, by Burr (1939), page 229. 

